It is known to produce a conductive sheet by mixing conductive amorphous carbon, graphite, metal powder, or the like with rubber or resin, and molding the mixture by extrusion, compression, rolling, or other techniques, or, alternatively, by depositing a conductive metal on the surface of a rubber or resin sheet by vacuum vapor deposition, sputtering deposition, etc.
However, taking the former method as an example, it is generally difficult to obtain a sufficient degree of electroconductivity, because there is unavoidably a certain limitation on the amount of a conductive substance that can be added in order that a mixture of rubber or resin with such conductive substance may be successfully molded into a sheet form. On the other hand, a wet process is also known for producing a porous film, comprising casting a solution of a film-forming resin in a water-miscrible organic solvent onto a suitable support and immersing the same in water. However, a similar limitation exists on the amount of the conductive substance that can be incorporated in this process as well, when such substance is to be incorporated in the resin solution, with the result that a sufficiently conductive porous film cannot be easily produced by such a process.
In the latter method, even if the surface of the sheet can be rendered conductive, the sheet is generally insulating in the thickness direction. Moreover, if one is to produce a flexible conductive sheet, the deposition thickness of the conductive metal must be limited in order that the flexibility of the support sheet may be retained, with the result that the electroconductivity of the product sheet is also limited to within a certain range.
Some of the products of the oxidative polymerization of aniline, e.g., aniline black, have long been known. Particularly, as an intermediate in the production of aniline black, the octamer of aniline represented by formula (I) has been identified as emeraldine (A. G. Green et al., J. Chem. Soc., Vol. 97, p. 2388 (1910), ibid., Vol. 101, p. 1117 (1912)). This octamer is soluble in 80% acetic acid, cold pyridine, and N,N-dimethylformamide. ##STR2## The emeraldine is oxidized in an ammoniacal medium to form nigraniline, represented by formula (II), which is also known to possess solubility similar to that of emeraldine. ##STR3##
It has been demonstrated by R. Buvet et al that the sulfate of emeraldine possesses high electroconductivity (J. Polymer Sci., C, Vol. 16, pp. 2931, 2943 (1967), ibid., Vol. 22, p. 1187 (1969).
It has been also demonstrated that an organic substance similar to emeraldine can be obtained by electro-oxidative polymerization of aniline (D. M. Mohilner et al., J. Amer. Chem. Soc., Vol. 84, p. 3618 (1962)). According to this publication, a substance soluble in 80% acetic acid, in pyridine, and in N,N-dimethylformamide can be obtained when an aqueous sulfuric acid solution of aniline is subjected to electro-oxidative polymerization using a platinum electrode at an oxidation potential of +0.8 V relative to the standard calomel electrode, a level necessary for avoiding electrolysis of water.
In addition to the reports mentioned above, Diaz et al. (J. Electroanal. Chem., Vol. 111, p. 111 (1980)) and Oyama et al. (Polymer Preprints, Japan, Vol. 30, No. 7, p. 1524 (1981), J. Electroanal Chem., Vol. 161, p. 339 (1984)) have also studied electro-oxidative polymerization of aniline. These latter studies were aimed at polymer coated chemically modified electrodes where the electrolysis is conducted at potentials not exceeding 1 V.
There are already known various electroconductive organic polymers, but as a general tendency these polymers are lacking in stability. For example, while polyacetylene is theoretically a very interesting electroconductive organic polymer, it is so susceptible to oxidation that it quickly undergoes oxidative degradation in air, considerably altering its properties. In the doped state, it is still more sensitive to oxidation, and its electroconductivity suffers a rapid decrease in the presence of even a small quantity of moisture in the atmosphere. This tendency is particularly pronounced in the case of n-type semiconductors.